Great advances have been achieved in the development and the implementation of fuel cells, in recent years. In the case of a solid polymer electrolyte fuel cell, for example, the fuel cell is constructed by bonding gas diffusion electrodes obtained by providing a porous carbon film composed of a carbon fiber sheet with a thickness of 0.1-0.3 mm, on the surface of which a platinum-based catalyst is supported as an electrode catalyst on both sides of a polymer solid electrolyte layer, and providing a separator, made of a dense carbon board with a thickness of 1-3 mm and having a gas flow channel on the outside of the porous carbon film, on each side.
In the case of a phosphoric acid-type fuel cell, for example, the fuel cell is constructed by bonding gas diffusion electrodes, obtained by providing a porous carbon film, composed of a carbon fiber sheet with a thickness of 0.1-0.3 mm and on the surface of which a platinum-based catalyst is supported as an electrode catalyst, on each side of an electrolyte layer obtained by supporting phosphoric acid on a phosphoric acid support, and providing a separator, made of a dense carbon board with a thickness of 1-3 mm and having a gas flow channel on the outside of the porous carbon film, on each side.
Powdered materials such as carbon black have conventionally been used as carbon materials for precious metal-based catalyst supports because they increase the supporting specific surface area. For application to fuel cell electrodes, however, resin binders with substantially no electron conductivity must be used for molding into film shapes (for example, Japanese Unexamined Patent Publication No. 5-36418), and this has led to such problems as increased internal resistance of the electrodes, poor in-plane uniformity of the reaction and, consequently, inferior battery properties.
The present inventors have previously proposed a porous carbon film which can retain a film-like shape without using a resin binder, and its application to fuel cell electrodes.
However, the important step of stirring in conventional metal dispersing supporting techniques employing metal precursor solutions is difficult to apply to porous carbon films, while it has been extremely difficult to support nanosize-scale metal fine particles in a uniform manner.
Moreover, metals commonly used for fuel cell electrodes, and especially platinum-based materials, are very expensive and, although it is desirable to achieve dispersion and support of fine particles (preferably 2-10 nm fine particles) in a uniform manner in order to maximize their activity per weight, no method has yet been achieved for achieving uniform dispersion while also accomplishing particle size control, and therefore it is currently the case that the supporting of metals, such as platinum, must be accomplished on the basis of experience and intuition.